The crust, when formed thus slowly, adheres with great firmness. The most careful analysis cannot detect any lead, either dissolved in the water, or floating in it, or united with the insoluble matter left on the side of the glass by evaporation. In short, the preservation of the lead from corrosion, and of the water from impregnation with lead, is complete.[1]
When the protection afforded is not quite complete,—for example in distilled water containing a 4000th of muriate of soda, a 6000th of sulphate of soda, a 15,000th of arseniate of soda, or a 35,000th of phosphate of soda,—besides a powdery crust, small crystals, with several facettes, are sometimes formed on the lead, while, at the same time, a minute white film will very slowly appear on the bottom of the glass, on its side where it is left dry by the evaporation of the water, and likewise on the surface of the water itself. These detached films are composed of carbonate of lead, with a little of the muriate, sulphate, arseniate, or phosphate of lead, according to the nature of the acid in the alkaline salt which is dissolved in the water. In the course of the changes now described, the lead in general no longer gains, but loses weight. The loss, however, is exceedingly small.—No lead can be discovered in solution, if the water before evaporation is carefully filtered.
On progressively trying solutions of weaker and weaker preservative power, it will be remarked, that the quantity of the detached powder, and the proportion of carbonate in it, progressively increase; and likewise, that what is formed on the lead adheres more and more loosely. In distilled water and weak solutions of acetate of soda, or nitrate of potass, the lead never becomes so firmly encrusted, but that gentle agitation of the water will shake off the powder.
It is worthy of notice that, although a small quantity of lead is dissolved by distilled water after it has remained some time in contact with the metal, yet not a trace is found in solution where a protecting salt is present. In solutions even weakly preservative I never could detect any lead dissolved. Thus, in distilled water containing a 4000th of muriate of soda, or a 160th of nitre, the lead lost weight, and loose crystals of carbonate were formed; yet even after thirty days no lead could be found in solution by the process with which I have always detected it in pure distilled water. Free exposure to the air is probably in part the cause of this. For it will be seen afterwards that some natural waters in passing through a long course of lead pipes, within which the action goes on without direct access of the atmosphere, contract an impregnation, which is invisible when the water is newly drawn, but after a few hours' exposure to the air shows itself in the form of a white film and milkiness.
The general result of these experiments appears to be, that neutral salts in various, and for the most part minute, proportions, retard
- ↑ Sometimes, however, a minute trace of white powder is attached to the bottom of the glass wherever the lead touches it. This is carbonate of lead at first, and afterwards a mixture like that described in the text.
